Electromagnetic sensors provide one way to determine the position and orientation of a portion of a medical instrument during a minimally invasive medical procedure. For example, a minimally invasive lung biopsy procedure may need to navigate a lung catheter through the airways or bronchia of a patient until a distal tip of the lung catheter reaches a target site such as a node in the patient's lungs. The lung catheter may include one or more electromagnetic sensors for measurements of the locations and orientations of lung catheter sections containing the electromagnetic sensors. Typically, each electromagnetic sensor includes one or more sensing coils that act as antennas and generate induced electrical signals when subjected to a magnetic field that varies with time. During a minimally invasive procedure, a field generator can be positioned near the patient to generate a magnetic field having a magnitude or direction or both that varies over time in a controlled manner. Analysis of the electrical signal induced in a single coil may provide measurements of multiple degrees of freedom, e.g., x, y, and z coordinates and pitch and yaw angles, of the coil or the portion of the instrument including the coil.
The strength of the induced signal from a coil in an electromagnetic sensor generally depends on the number of windings in the coil, the area of each winding, and the magnetic permeability of a core running through the windings as well as the magnitude and direction of changes in the magnetic field. For minimally invasive medical instruments, the diameters of the coils need to be small when the diameter of the instrument is small, e.g., to navigate into smaller airways. As a result, the coils in electromagnetic sensors are generally long to allow many windings and generally employ a core made of a material with a high magnetic permeability, e.g., a material with a relative magnetic permeability of about 10 or more. The length and rigidity of a coil in an electromagnetic sensor can limit the flexibility of the instrument in which the electromagnetic sensor is mounted.